History of Glaciations
See also: Timeline of Glaciations An ice age is a long period of reduction in the temperature of the 's surface and atmosphere, resulting in the presence or expansion of continental and polar s and alpine s. Earth is currently in the , known in popular terminology as the Ice Age. Individual pulses of cold climate are termed " s" (or, alternatively, "glacials", "glaciations", "glacial stages", "stadials", "stades", or colloquially, "ice ages"), and intermittent warm periods are called " s" or "interstadials" with both climatic pulses part of the or . In the terminology of , ice age implies the presence of extensive ice sheets in both northern and southern hemispheres. By this definition, we are in an interglacial period—the . The amount of heat trapping gases emitted into Earth's Oceans and atmosphere will prevent the next ice age, which otherwise would begin in around 50,000 years, and likely more glacial cycles. Major ice ages glacial; yellow: glacial at maximum (Drenthe stage); blue: glacial maximum glaciation.}} .}} There have been at least five major ice ages in the Earth's history (the , , , , and the latest ). Outside these ages, the Earth seems to have been ice free even in high latitudes. Rocks from the earliest well established ice age, called the , formed around 2.4 to 2.1 ( years) ago during the early Eon. Several hundreds of km of the are exposed 10–100 km north of the north shore of Lake Huron extending from near Sault Ste. Marie to Sudbury, northeast of Lake Huron, with giant layers of now-lithified till beds, dropstones, varves, outwash, and scoured basement rocks. Correlative Huronian deposits have been found near , and correlation has been made with Paleoproterozoic glacial deposits from Western Australia. The Huronian ice age was caused by the elimination of atmospheric , a , during the . The next well-documented ice age, and probably the most severe of the last billion years, occurred from 720 to 630 million years ago (the period) and may have produced a in which glacial ice sheets reached the equator, possibly being ended by the accumulation of es such as produced by volcanoes. "The presence of ice on the continents and pack ice on the oceans would inhibit both and , which are the two major sinks for at present." It has been suggested that the end of this ice age was responsible for the subsequent and , though this model is recent and controversial. The occurred from 460 to 420 million years ago, during the and the period. The evolution of land plants at the onset of the period caused a long term increase in planetary oxygen levels and reduction of levels, which resulted in the . Its former name, the Karoo glaciation, was named after the glacial tills found in the Karoo region of South Africa. There were extensive polar s at intervals from 360 to 260 million years ago in South Africa during the and Periods. Correlatives are known from Argentina, also in the center of the ancient supercontinent . The icing of Antarctica began in the middle about 45.5 million years ago and escalated during the about 34 million years ago. CO2 levels were then about 760 ppm. The opening of the Drake Passage may have played a role. See 40 km (25 mi), and 100 km (62 mi). The started about 2.58 million years ago at the beginning of the when the spread of ice sheets in the Northern Hemisphere began. Since then, the world has seen cycles of glaciation with ice sheets advancing and retreating on 40,000- and 100,000-year time scales called s, glacials or glacial advances, and periods, interglacials or glacial retreats. The earth is currently in an interglacial, and the last glacial period ended about 10,000 years ago. All that remains of the continental s are the and s and smaller glaciers such as on . The definition of the as beginning 2.58 Ma is based on the formation of the . The began to form earlier, at about 34 Ma, in the mid- ( ). The term is used to include this early phase. Ice ages can be further divided by location and time; for example, the names Riss (180,000–130,000 years ) and (70,000–10,000 years bp) refer specifically to glaciation in the . The maximum extent of the ice is not maintained for the full interval. The scouring action of each glaciation tends to remove most of the evidence of prior ice sheets almost completely, except in regions where the later sheet does not achieve full coverage. Glacials and interglacials | image2 = Iceage south-intergl glac hg.png | caption2 = Minimum (interglacial, black) and maximum (glacial, grey) glaciation of the }} Within the ice ages (or at least within the current one), more temperate and more severe periods occur. The colder periods are called glacial periods, the warmer periods interglacials, such as the . Glacials are characterized by cooler and drier climates over most of the earth and large land and sea ice masses extending outward from the poles. Mountain glaciers in otherwise unglaciated areas extend to lower elevations due to a lower . Sea levels drop due to the removal of large volumes of water above sea level in the icecaps. There is evidence that ocean circulation patterns are disrupted by glaciations. Since the earth has significant continental glaciation in the Arctic and Antarctic, we are currently in a glacial minimum of a glaciation. Such a period between glacial maxima is known as an interglacial. The glacials and interglacials also coincided with changes in Earth's orbit called . The earth has been in an interglacial period known as the for around 11,700 years, and an article in Nature in 2004 argues that it might be most analogous to a previous interglacial that lasted 28,000 years. Predicted changes in suggest that the next glacial period would begin at least 50,000 years from now, due to the . Moreover, anthropogenic forcing from increased es is estimated to potentially outweigh the orbital forcing of the Milankovitch cycles for hundreds of thousand of years. Recent glacial and interglacial phases of about 120 m.}} ice core for the past 420,000 years}} The entire Quaternary Period, starting 2.58 Ma, is referred to as an ice age because at least one permanent large ice sheet—the —has existed continuously. There is uncertainty over how much of was covered by ice during each interglacial. The is marked by warm and cold episodes, cold phases called ( ) lasting about 100,000 years, and which are then interrupted by the warmer s which lasted about 10,000–15,000 years. Initially the fluctuation period was about 41,000 years, but following the it has slowed to about 100,000 years, as evidenced most clearly by s for the past 800,000 years and marine sediment cores for the earlier period. Over the past 740,000 years there have been eight glacial cycles. The last cold episode of the ended about 10,000 years ago. Earth is currently in an interglacial period of the Quaternary, called the . This caused the ice sheets from the to . Remnants of these last glaciers, now occupying about 10% of the world's land surface, still exist in Greenland, Antarctica and some mountainous regions. Glacial stages in North America The major glacial stages of the current ice age in North America are the , and . The use of the Nebraskan, Afton, Kansan, and Yarmouthian stages to subdivide the ice age in North America has been discontinued by Quaternary geologists and geomorphologists. These stages have all been merged into the in the 1980s. During the most recent North American glaciation, during the latter part of the (26,000 to 13,300 years ago), ice sheets extended to about . These sheets were 3-4 km thick. development in the region of the current North American .}} This Wisconsin glaciation left widespread impacts on the North American landscape. The and the were carved by ice deepening old valleys. Most of the lakes in Minnesota and Wisconsin were gouged out by glaciers and later filled with glacial meltwaters. The old drainage system was radically altered and largely reshaped into the drainage system. Other rivers were dammed and diverted to new channels, such as , which formed a dramatic waterfall and gorge, when the waterflow encountered a limestone escarpment. Another similar waterfall, at the present near , is now dry. The area from to was formed from glacial , and the plethora of lakes on the in northern Canada can be almost entirely attributed to the action of the ice. As the ice retreated and the rock dust dried, winds carried the material hundreds of miles, forming beds of many dozens of feet thick in the . continues to reshape the Great Lakes and other areas formerly under the weight of the ice sheets. The , a portion of western and southwestern Wisconsin along with parts of adjacent , , and , was not covered by glaciers. Last Glacial Period in the semiarid Andes around Aconcagua and Tupungato A specially interesting climatic change during glacial times has taken place in the semi-arid Andes. Beside the expected cooling down in comparison with the current climate, a significant precipitation change happened here. So, researches in the presently semiarid subtropic Aconcagua-massif (6,962 m) have shown an unexpectedly extensive glacial glaciation of the type "ice stream network". The connected valley glaciers exceeding 100 km in length, flowed down on the East-side of this section of the Andes at 32–34°S and 69–71°W as far as a height of 2,060 m and on the western luff-side still clearly deeper. Where current glaciers scarcely reach 10 km in length, the snowline (ELA) runs at a height of 4,600 m and at that time was lowered to 3,200 m , i.e. about 1,400 m. From this follows that—beside of an annual depression of temperature about c. 8.4 °C— here was an increase in precipitation. Accordingly, at glacial times the humid climatic belt that today is situated several latitude degrees further to the S, was shifted much further to the N. Effects of glaciation exhibits some of the typical effects of ice age glaciation such as s and lakes.}} Although the last glacial period ended more than 8,000 years ago, its effects can still be felt today. For example, the moving ice carved out the landscape in Canada (See ), Greenland, northern Eurasia and Antarctica. The s, , s, s, s, s, s, s, , etc., are typical features left behind by the glaciers. The weight of the ice sheets was so great that they deformed the Earth's crust and mantle. After the ice sheets melted, the ice-covered land . Due to the high of the , the flow of mantle rocks which controls the rebound process is very slow—at a rate of about 1 cm/year near the center of rebound area today. During glaciation, water was taken from the oceans to form the ice at high latitudes, thus global sea level dropped by about 110 meters, exposing the continental shelves and forming land-bridges between land-masses for animals to migrate. During , the melted ice-water returned to the oceans, causing sea level to rise. This process can cause sudden shifts in coastlines and hydration systems resulting in newly submerged lands, emerging lands, collapsed resulting in of lakes, new ice dams creating vast areas of freshwater, and a general alteration in regional weather patterns on a large but temporary scale. It can even cause temporary . This type of chaotic pattern of rapidly changing land, ice, saltwater and freshwater has been proposed as the likely model for the and n regions, as well as much of central North America at the end of the last glacial maximum, with the present-day coastlines only being achieved in the last few millennia of prehistory. Also, the effect of elevation on Scandinavia submerged a vast continental plain that had existed under much of what is now the North Sea, connecting the British Isles to Continental Europe. The redistribution of ice-water on the surface of the Earth and the flow of mantle rocks causes changes in the as well as changes to the distribution of the of the Earth. These changes to the moment of inertia result in a change in the , , and wobble of the Earth's rotation. The weight of the redistributed surface mass loaded the , caused it to and also induced within the Earth. The presence of the glaciers generally suppressed the movement of below. During , the faults experience accelerated slip triggering s. Earthquakes triggered near the ice margin may in turn accelerate and may account for the . As more ice is removed near the ice margin, more s are induced and this positive feedback may explain the fast collapse of ice sheets. In Europe, glacial erosion and sinking from weight of ice made the , which before the Ice Age was all land drained by the . Lakes The Quaternary glaciation created more lakes than all other geologic processes combined. The reason is that a continental completely disrupts the preglacial . The surface over which the glacier moved was scoured and by the ice, leaving a myriad of closed, undrained depressions in the bedrock. These depressions filled with water and became lakes. Very large lakes were created along the glacial margins. The ice on both and was about thick near the centers of maximum accumulation, but it tapered toward the glacier margins. Ice weight caused crustal subsidence, which was greatest beneath the thickest accumulation of ice. As the ice melted, rebound of the crust lagged behind, producing a regional slope toward the ice. This slope formed basins that have lasted for thousands of years. These basins became lakes or were invaded by the ocean. The and the of North America were formed primarily in this way. The numerous lakes of the , Sweden, and are thought to have originated at least partly from glaciers' selective erosion of . Pluvial lakes The climatic conditions that cause glaciation had an indirect effect on arid and semiarid regions far removed from the large s. The increased precipitation that fed the s also increased the runoff of major rivers and intermittent streams, resulting in the growth and development of large pluvial lakes. Most pluvial lakes developed in relatively arid regions where there typically was insufficient rain to establish a drainage system leading to the sea. Instead, stream runoff flowed into closed basins and formed s. With increased rainfall, the playa lakes enlarged and overflowed. Pluvial lakes were most extensive during glacial periods. During interglacial stages, with less rain, the pluvial lakes shrank to form small salt flats. Isostatic adjustment Major isostatic adjustments of the during the Quaternary glaciation were caused by the weight of the ice, which depressed the continents. In , a large area around was depressed below (modern) sea level, as was the area in Europe around the Baltic Sea. The land has been rebounding from these depressions since the ice melted. Some of these isostatic movements triggered large s in Scandinavia about 9,000 years ago. These earthquakes are unique in that they are not associated with . Studies have shown that the uplift has taken place in two distinct stages. The initial uplift following was rapid (called "elastic"), and took place as the ice was being unloaded. After this "elastic" phase, uplift proceed by "slow viscous flow" so the rate decreased after that. Today, typical uplift rates are of the order of 1 cm per year or less. In northern Europe, this is clearly shown by the data obtained by the BIFROST GPS network. Studies suggest that rebound will continue for about at least another 10,000 years. The total uplift from the end of deglaciation depends on the local ice load and could be several hundred meters near the center of rebound. Winds The presence of ice over so much of the continents greatly modified patterns of atmospheric circulation. Winds near the glacial margins were strong and persistent because of the abundance of dense, cold air coming off the glacier fields. These winds picked up and transported large quantities of loose, fine-grained sediment brought down by the glaciers. This dust accumulated as (wind-blown silt), forming irregular blankets over much of the valley, central Europe, and northern China. Sand s were much more widespread and active in many areas during the early Quaternary period. A good example is the region in , USA, which covers an area of about 60000 km2. This region was a large, active dune field during the epoch, but today is largely stabilized by grass cover. Ocean currents Thick glaciers were heavy enough to reach the sea bottom in several important areas, thus blocking the passage of ocean water and thereby affecting ocean currents. In addition to direct effects, this caused feedback effects as ocean currents contribute to global heat transfer. Next glacial period since the .}} The warming trend following the , since about 20,000 years ago, has resulted in a by about 130 metres. This warming trend has subsided about 6,000 years ago, and sea level has been comparatively stable since the . The present interglacial period (the ) has been fairly stable and warm, but the previous one was interrupted by numerous cold spells lasting hundreds of years. If the previous period was more typical than the present one, the period of stable climate, which allowed the and by extension human , may have been possible only because of a highly unusual period of stable temperature. Based on , the cooling trend initiated about 6,000 years ago will continue for another 23,000 years. Slight changes in the Earth's orbital parameters might however indicate that, even without any human contribution, there will not be another glacial period for the next 50,000 years. It is possible that the current cooling trend may be interrupted by an in about 60,000 years, with the next glacial maximum reached only in about 100,000 years. Based on past estimates for interglacial durations of about 10,000 years, in the 1970s there was some concern that the next glacial period would be imminent. However, slight changes in the eccentricity of Earth's orbit around the Sun suggest an extended interglacial for about 50,000 years. Additionally, is now seen as possibly delaying what would already be an unusually long warm period. Projection of the timeline for the next glacial maximum depend crucially on in the atmosphere}}. Models assuming increased levels at 750 parts per million ( ; current levels are at 407 ppm) have estimated the persistence of the current interglacial period for another 50,000 years. However, more recent studies concluded that due to the amount of heat trapping gases emitted into Earth's Oceans and atmosphere, that this will prevent the next glacial (ice age), which otherwise would begin in around 50,000 years, and likely more glacial cycles. References Category:History of the world